Epoxy-based nano-composites are known to have properties that are superior over neat epoxy, which is a brittle resin with poor resistance to crack initiation.
Toughening strategies have included the incorporation of filler particles using such materials as rubber particles, core-shell particles, hydro-branched polymer particles, thermoplastic particles and inorganic particles. The selection of filler particles has an impact on the nano-composites mechanical properties, such as strength, modulus and, especially in the case of epoxy-based nano-composites, their thermal stability.
Inorganic filler particles are commonly used modifiers for nano-composite production because of their ability to improve toughness while maintaining high modulus and thermal stability. The volume fraction, dispersion, size, type and surface functionalization of inorganic fillers play important roles in the mechanical reinforcement of the polymer. The properties of composites can be significantly improved when micron-sized fillers are replaced by nano-sized fillers, due to the reduction in the inter-particle distances. This is because stress transfer from matrix to filler, suppression of polymer chain mobility and the amount of energy dissipation at crack initiation and propagation can be promoted by the presence of mechanically coupled networks at low inter-particle distance.
Sol-gel silica, which has low thermal expansion, uniform morphology and particle size, is a known nano-filler for nanocomposite preparation. The extent of silica nanoparticles dispersion within an epoxy matrix and the interfacial interaction between the silica nanoparticles and the epoxy matrix are important parameters affecting properties of the silica-epoxy nanocomposite formed. Currently known methods attempt to improve interfacial interaction between silica nanoparticles and epoxy matrix through functionalization of silica surfaces by amine-terminated coupling agent prior to dispersion into the epoxy resin. One major disadvantage of using such sol-gel route in nanocomposite preparation is the use of solvent in the preparation process. The solvent needs to be removed and recycled or disposed of, thus posing environmental, health and safety issues, in addition to the additional costs involved in solvent removal or disposal. As a result of the high production costs due to complexity of the manufacturing process, silica nanocomposite derived from the sol-gel process is particularly limited to high tech applications, such as aerospace, automotive and electronics.
In situ sol-gel silica/epoxy preparation methods have also been developed to simplify the preparation of nanocomposites. Such methods include in situ sol-gel synthesis of silicon alkoxide under vacuum or elevated temperature. Although the process of silica/epoxy nanocomposite production has been simplified through the combination of silica formation step with nanocomposite preparation, this method is not widely adopted in industry because of the non-uniformity of silica nanoparticles produced, poor silica dispersion and weak interfacial interaction between the silica and epoxy matrix. In addition, a solvent is still required in this process as a diluent and therefore issues regarding solvent removal or disposal remain relevant.
For both sol-gel and in situ sol-gel methods, very high silica content (6-30% by weight) in silica/epoxy nanocomposite is required to achieve high mechanical and thermal properties. Preparation of composites with lower degrees of silica content, while still maintaining its mechanical and thermal properties, is required for reduction of processing cost, because these silica sources (sol-gel silica, concentrated silica/epoxy resin, colloidal silica) are expensive.
For industrial-scale applications, resin transfer molding (RTM), resin infusion molding (RIM) and vacuum bagging processes are widely used. The these processes require that the nanocomposites be easily and completely fed into cavities of designed mold shape before curing. As such, high performance composites with low viscosity are preferred. However, viscosities of materials with very high silica content (6-30% by weight) are desired for their mechanical and thermal properties, thereby posing difficulties during processing, limiting their usage in industrial-scale applications.
Another method of forming a silica-epoxy nanocomposite requires the use of hexahydrophthalic anhydride in a mixture of diglycidyl ethers of bisphenol-A and tetraethoxysilane. In this process, the tetraethoxysilane has to go through a two step modification in order to form the silica nanoparticles, which tend to be spherical in shape. Further, this method requires the use of an autoclave to increase the temperature to 170° C., which is not cost-efficient.
Therefore, there is a need for a method for preparing a nanocomposite that overcomes, or at least ameliorates one or more of the disadvantages described above.
There is also a need to provide a method of producing a silica/epoxy nanocomposite that possesses superior properties, such as improved mechanical and thermal properties over neat epoxy and which can be produced on an industrial scale.